Update realloc_raw docstring

src/arena_allocator.hpp

@@ -237,6 +237,10 @@ public:
    * - The old pointer becomes invalid if a copy occurs
    * - Like malloc/realloc, the contents beyond old_size are uninitialized
    * - When copying to new location, uses the specified alignment
+   * - **Shrinking behavior**: If `new_size < old_size` and the allocation
+   *   is *not* the most recent one, this function returns the original
+   *   pointer, but **no memory is reclaimed**. The arena design does not
+   *   support freeing memory from the middle of a block.
    */
   void *realloc_raw(void *ptr, uint32_t old_size, uint32_t new_size,
                     uint32_t alignment = alignof(std::max_align_t));